Abstract C56: Characterization of PinX1 as a coregulator of steroid hormone receptors

Abstract

Abstract Introduction: Deregulated expression of transcriptional coactivators and corepressors has been implicated in tamoxifen (TAM) resistance, especially in ER+ breast cancer patients. The AF-1 transcriptional activation domain in the N-terminus of ER has been implicated in the tissue specific agonistic effects of TAM. Thus, using the yeast two-hybrid system with the N-terminal region of ERα, we located the PINX1 gene which encodes for a telomerase inhibitor. Objective: In this work, we characterized PinX1 as a coregulator of ERα. Methods and Results: Using GST pull-down assays, we confirmed the direct interaction between PinX1 and the N-terminus of ERα, showing no interaction with the C-terminus either in the presence or absence of estradiol (E2). Transcriptional activation assays using luciferase reporter genes show that PinX1 is capable of increasing ERα transcriptional activity in breast cancer cell lines, at low concentrations, but increasing PinX1 exhibits a negative effect on the transcriptional activity of ERα in these cell lines. However, Pinx1 repressed AF-1 transcriptional activity, as well as AF-2 activity in the presence of E2. ChIP assays verified that the interaction between ERα and Pinx1 occurs on E2 regulated promoters after 2h of incubation with E2. Furthermore, PinX1 inhibits E2 induced proliferation of MCF7 cells, while inhibition of PinX1 by siRNA increases E2 induced proliferation. These results suggest a role for PinX1 as a correpresor of ERα. PinX1 also interacts with the androgen and progesterone receptors. However, we demonstrate that PinX1 is a coactivator of these receptors by luciferase assays, as well as increasing DHT induced proliferation of prostate cancer cell lines. Enhanced expression of PinX1 also deregulates the expression of a number of genes that have a role in cell growth and proliferation in breast cancer cell lines. Conclusions: Our data support a dual role for PinX1 as a correpresor of ERα and a coactivator of AR and PR. Additionally, it may be involved in functions regulating cell growth and proliferation in both breast and prostate cancer cell lines. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr C56.